A conventional speaker device of the same type is described with reference to FIG. 7, FIG. 8 (A) and FIG. 8 (B). FIG. 7 is a block diagram, FIG. 8 (A) shows microphone output signals, FIG. 8 (B) shows sound output characteristics of the conventional speaker device, where, curve “a” shows a sound pressure characteristic, and curve “b” shows a phase characteristic.
Referring to FIG. 7, a speaker unit 1 generates sound waves, and the speaker unit 1 is coupled with an acoustic pipe 2. At both sides of the acoustic pipe 2 are sound absorbing material (not shown) provided for suppressing resonance. Inside the acoustic pipe 2, a microphone 4 is provided close to the speaker unit 1 for detecting sound output signals.
When a signal is delivered to the speaker unit 1 via a subtracter 6 and a power amplifier 3, the speaker unit 1 radiates acoustic output, which is radiated from the opening through the acoustic pipe 2. The standing wave due to the length of the acoustic pipe 2 and the one generated within the acoustic pipe 2 causes a speaker device to reproduce sounds having steep peaks and dips in the sound pressure frequency characteristic. In order to prevent this, a sound absorbing material is employed to suppress the standing waves. However, the sound absorbing material is not effective enough to suppress the standing waves completely. So, a microphone 4 detects the remaining standing wave and feeds it back to the subtracter 6 via a microphone amplifier 5. Thus, the standing wave in acoustic pipe 2 is suppressed, and the reproduced sounds with a flat sound pressure frequency characteristic are obtained.
An acoustic pipe coupled in the front of a speaker unit is known to produce a resonance in the pipe; the resonance frequency f generated is represented by the formula below:f=(n+1)C/4L 
where; f: pipe resonance frequency, n: the n-th resonance, C: sound velocity, L: length of the pipe.
In the above configured speaker device, when a primary resonance (n=1) due to the pipe length is corrected by means of the phase difference between the electrical input signal delivered to the speaker unit 1 and the sound output signal radiated from the speaker unit 1, the resonance component shifts and appears as a peak in the sound output characteristic after the correction. So, it has been difficult to flatten the sound output characteristic. Furthermore, since the feedback is performed for an entire frequency range from a low frequency component to a high frequency component, it is impossible to control a certain desired frequency component.
The relationship between the input and the output is shown below:V out/V in=A/(1+A·T(S))
where; V out: output voltage, V in: input voltage, A: total amplification by amplifiers, T (S) transfer function.
Assuming that the microphone 4 has an approximately flat characteristic and the T (S) is substantially equal to the transfer function of the speaker unit 1, the T (S) becomes minus 1 as a result of phase shift caused by the speaker unit 1 and the second, or the third, pipe resonance of acoustic pipe 2.
Namely, in some cases the denominator becomes 0 to be ready for making oscillation. This makes it difficult to apply too many feedbacks taking an oscillation margin into consideration, and to effectively control a low frequency region and a pipe resonance.
The present invention addresses the above problems and aims to provide a speaker device that has stable characteristics.